A receiver input signal, such as a Radio Frequency (RF) signal, has to be frequency-shifted (“down-converted”) to be centred around Direct Current (DC), or zero frequency, before the signal content can be retrieved. For down-conversion, the RF signal is conventionally shifted to an Intermediate Frequency (IF) for initial signal processing before it is further shifted to DC. Modern receivers are often implemented as direct conversion receivers that avoid the first signal shift to IF. For this reason such receivers are also called zero-IF receivers.
While zero-IF receivers are advantageous as regards power, cost and size considerations, their characteristics are partially inferior to IF-based receivers. It has, for example, been observed that zero-IF receivers often suffer from an imbalance between different signal branches. The imbalance can be attributed to gain and phase errors introduced by components provided separately in or for the different signal branches such as mixers and oscillators. As an example of a signal branch imbalance, the imbalance between In-phase (I) and Quadrature (Q)-phase signal branches can be mentioned. This imbalance is also referred to as IQ imbalance.
The signal branch imbalance may have a frequency-independent or a frequency-dependent (i.e., frequency-selective) characteristic. Frequency-dependent imbalances have become an important factor to consider and mitigate as transmission bandwidth and modulation order are increasing.
US 2005/0260949 A discloses a technique for compensating a frequency-dependent IQ imbalance introduced by different characteristics of filters individually provided in the I and Q signal branches of a receiver. To this end, a test signal is applied to the filter inputs. The test signal is composed of two tones (i.e., two superimposed frequencies). The first tone is positioned near the centre of a filter pass-band, while the second tone is positioned near the edge of the pass-band. The filtered test signals are converted to the frequency domain, and gain and phase errors indicative of the IQ imbalance are determined next from the resulting frequency domain representations. In a final step, the IQ imbalance is compensated for based on the gain and phase errors thus determined.
As an alternative to a test signal composed of two tones, US 2005/0260949 A also teaches the utilization of one-tone test signals. Specifically, the first tone may be injected into the I and Q signal branches during a first time interval and the second tone may be injected during a subsequent second time interval. Still further, US 2005/0260949 A mentions that testing may also be performed over a large number of tones to determine an error matrix.